Re: Really, *really* big pterosaurs?

[Michael Habib <mhabib5@jhmi.edu>]

On Friday, September 9, 2005, at 07:03 PM, Patrick Norton wrote:

> Impressive. I wonder what the size limits might be for vertebrate
> soarers? The Reuters article suggests a very thin airfoil and very 
> light
> skeleton in this animal, which makes sense, but how far can the
> vertebrate soaring bauplan be extended in this regard? I think it's
> fairly well accepted that powered, flapping flyers are limited to
> wingspans less than half this width (Argentavis sp), but what are the
> size/mass limits for vertebrate soarers?


Good question, and I am sure you would get various answers depending on 
who you asked.  Incidentally, the limitations on powered flight 
probably have more to do with mass-specific power limitations (as 
animals get larger, they get less bang per unit muscle weight) than it 
does with wingspan.  Especially large fliers tend to have 
disproportionately long wings because large fliers tend to utilize 
soaring a great deal (note that oceanic soarers, like albatrosses, are 
built very differently from terrestrial soarers like condors).  This is 
more complicated than it seems at first, because while part of the 
story is that large fliers are 'limited' somewhat to soaring by virtue 
of size, it is also true that large fliers often make excellent 
soarers.  That is, there is some advantage to being big if an animal is 
adapted to soaring (example: wind penetration for oceanic, dynamic 
soarers like albatrosses).  Since prolonged soaring is very efficient 
(and, in some cases, quite a derived form of flight), the adaptation of 
vertebrate fliers to soaring-oriented flight may not always be simply 
do to limitations by weight, and similarly, some species may be larger 
because they utilize soaring.  To put it another way, just because a 
flying animal is large and soars, does not mean it is above the weight 
limit for powered flight, nor does it mean that soaring evolved in that 
lineage because of size constraints.

For very large, soaring pterosaurs, one constraint might actually be 
their bone properties.  In order to be both light and strong, the bones 
of pterosaur arms and fingers (specifically the first digit) are 
relatively wide, but have very thin cortices.  At large sizes, 
pterosaur bones become extremely thinly walled (presumably for weight 
reduction, which, due to allometry, gets more 'difficult' at larger 
sizes), such that they are much more likely to fail by having low 
stiffness than because of low strength (Currey, 2002), which means that 
stiffness would be the limiting factor for bone design in this case.  
(Stiffness=ability to deform to load and return to original shape).  In 
other words, a gigantic pterosaur would need to have a very low mass 
relative to volume, which would require bones that had very low 
stiffness, and were likely to fail by flexing past their ability to 
recover.  With a large number of good body mass estimates and bone 
cross-sections, this relationship could be worked out, but it would 
only be useful for calculating a maximum size if combined with some 
knowledge of the forces acting on a pterosaur wing.